One of the most important tasks for the filter-compensation devices is an accurate determination of changing harmonic components of a non-sinusoidal signal. It helps the power component of the filter compensate the appropriate harmonics. The form of the network currents and voltages in the autonomous electric energetic systems operating with powerful semiconductor load are rather distorted and generator sets generate voltage with non-constant frequency, phase and amplitude. Thus, the calculation of the harmonic composition of the distorted signals proves to be a sophisticated problem.